Measurement of molecules and particles (simply denoted particles hereafter) by light scattering methods is frequently preferred over other methods because light scattering provides the advantages of convenient, fast, sensitive, non-destructive, in situ or even in vivo measurement. However, the inference of particle properties from static light scattering (SLS) signals can be extremely complex, depending strongly on particle index, size, shape and homogeneity, properties which are not generally known a priori. Measurement of particles by dynamic light scattering (DLS) techniques retains the advantages listed above while eliminating complexity in inferring properties including particle size and shape, since these inferences do not require detailed knowledge of the optical properties of the particles.
One strength of DLS techniques is their ability to measure a weighted mean value of a dynamic particle property (translational or rotational diffusion coefficient, electrophoretic mobility, . . . ) for a suspension of particles. However, this strength is sometimes a liability as the weighted mean value accessible is frequently not the one of interest and, except in special cases, the latter cannot be obtained from the former because these methods do not provide the distribution of particles over any property. Were a distribution provided, a number of moments of the distribution or the mean value of a number of properties of the suspension could be calculated.
Consider photon correlation spectroscopy (PCS) as an example of a DLS method. A PCS measurement of a suspension of spherical particles illuminated by a coherent light source provides, after some manipulation, the autocorrelation function of the electric field of the light signal scattered from the suspension. This function has the form of the Laplace transform of the product A(.GAMMA.)F(.GAMMA.) ##EQU1## where .tau. is the delay time, .GAMMA.=K.sup.2 D the linewidth variable for a particle having diffusion coefficient D=kT/f, k the Boltzmann's constant, T the absolute temperature, f=3.pi..eta.d the particle's friction coefficient, .eta. the viscosity of the medium, d the particle diameter, K the magnitude of the scattering vector, A(.GAMMA.) the light scattering cross-section and F(.GAMMA.) the number distribution density (per unit .GAMMA.) of particles of linewidth .GAMMA..
In principle, measured autocorrelation functions together with Equation (1) can be used to provide estimates of F(.GAMMA.) which can be transformed into a size or other distribution function. However, precise extraction of A(.GAMMA.)F(.GAMMA.) from Equation (1) is not trivial since this Fredholm integral equation of the first kind has the property that the A(.GAMMA.)F(.GAMMA.) extracted is very sensitive to noise in C(.tau.). Moreover, because A(.GAMMA.) is strongly dependent on particle size, the signal from any suspension of particles having significant breadth in its size distribution will be dominated by the signal from the fraction of particles having large A(.GAMMA.) while the remaining particles contribute only slightly to C(.tau.). Finally, C(.tau.) dependence on other properties such as particle shape can also contribute to the imprecision with which a one-dimensional distribution can be determined since such dependence can lead to (apparent) noise in C(.tau.) and to the non-uniform weighting of A(.GAMMA.)F(.GAMMA.) in Equation (1).
Because of these fundamental limitations in the PCS method in particular and in many DLS methods in general, improved methods are perused. The method of the present invention involves modulated dynamic light scattering (MDLS). In this method, a suspension of particles may be precisely characterized by individual measurement of many particles which provides precise distributions over one or more particles properties, singly or jointly. This strategy removes the limitations on precision associated with the inversion of a Fredholm integral equation, the simultaneous measurement of strong and weak signals and the measurement of particles distributed in an undetermined manner over additional properties. However, the methodology introduces its own limitations, namely, the longer time required to individually measure a large number of particles and the inability to measure particles that are too small to be individually detected. In spite of these limitations, the MDLS method will often provide more complete and precise information.